Immersion lithography at 193 nm has emerged as a useful technique for improving lithographic performance in a photoresist film by effectively increasing the numerical aperture (NA) of the exposure tool, thereby increasing the resolution that can be obtained from the photoresist. In immersion lithography, a film of water is in contact with the optics and the surface of a top coat coated on a photoresist film to be exposed. Photoresist formulations used in the film may, however, suffer from surface loss where some of the top coat may cause an unpredictable degree of top stripping, attributable to the strong interaction of acidic components in the top coat with acid sensitive components of the photoresist film.
To overcome this, surface active quenchers (i.e., bases) have been included in photoresist formulations. Excess base quencher in the surface layer of the photoresist film would mitigate the acid attack from the top coat and thus reduce the degree of top loss and help maintain good feature profile. Surface active quencher materials that have been used to enrich the surface layer of a photoresist film include fluorinated quencher bases and the base with long alkyl chains (C8 or longer).
While such an approach has demonstrated some improvements in reduced top loss, adverse effects such as decreased lithographic performance and increased micro-bridge defects may occur. This is believed to be due to the hydrophobic nature of the surface active quencher materials, which are essentially insoluble in aqueous base developer or water. Furthermore, small quencher molecules may be readily washed away during the top coat process since such quencher materials are generally soluble in the solvent used to apply the top coat.
It is therefore desirable to have a photoresist formulated so that there is a concentrated quencher material distributed on the film surface and the region near the top surface, which does not adversely affect lithographic and development performance.